THE COXSTITUTIOX OF CERTAIN ROCKS OF THE ARCH.T-AX AGE. 257' 



of the quartz and felspar appear in somewhat circular blebs. A 

 fact worthy of note is this, that \\here the hornblende crystals in 

 the sections have been cut parallel to their vertical axis the ver- 

 tical axes all appear to lie in parallel lines as if following' the line 

 of fluxion or flow. Occasionally a quartz crystal may show a 

 trace of straining-. The quartz does not contain liquid inclusions 

 to the extent it does in the other varieties of rocks mentioned. 

 The felspar appears to be the variety Labradoritc, with traces of 

 cleavage cracks, along \\hich decomposition has in many instances- 

 commenced. It is worthy of note that both the quartz and felspar 

 contain well-defined crystal inclusions of apatite and zircons. 

 Leaving this type for the present, I will meanwhile touch upon 

 the second type of intrusive rock, or^ as it may well be styled, the 

 \ounger series. This rock is holocrystalline in structure and non-^ 

 porphyritic. It consists of an admixture of triclinic felspar, augite 

 and other accessory minerals. The felspars are columnar and 

 show elongated sections, with no law of arrangement, and around 

 or between them the augite is moulded. It represents the Ophitic 

 structure typically. There is a fair quantity of magnetite pre- 

 sent and a small quantity of ilmenite, which is associated with its- 

 usual white decomposition product, viz., Leucoxene. The mag- 

 netite has also suffered through decomposition, and thus many 

 cracks and fissures, which traverse both the augite as well as the- 

 felspars in diverse directions, are filled with iron oxide stains 

 which radiate from the decomposed particles of magnetite. The- 

 magnetitite is found in crystals as well as granules. Where the 

 grains or crystals of magnetite have been small they have almost 

 entirely disappeared and left a reddish brown stain of iron oxide 

 on the spot where they once were, and have also filled up the 

 cleavage cfacks in the vicinity of the somewhat circular spot of 

 decomposition product. At times a piece of partially decomposed' 

 magnetite is to be seen in or about the centre of the spot. The 

 augite shows little or no pleochroism and has the usual cleavage 

 angles of 87° and 93° or thereabouts. The augite shows- 

 good cleavage planes, and is on the whole not badly decomposed. 

 The felspars show the albite type of twinning. Twinning on the 

 pericline type is less common, and an occasional instance occurs 

 of a felspar t^vinned on the Carlsbad type. The triclinic felspars 

 present belong to the basic end of the plagioclase series of fels- 

 pars. The principal variety is labradorite. This rock then ma}" 

 be regarded as a diabase. Let us now look at the rocks which 

 occur in this belt and that lie nearest the coastal side of it. They 

 extend from this central zone to almost the coast, where they are 

 overlain by the Table Mountain Series and other formations. 

 They outcrop at intervals and thus extend for some miles. To 

 outward appearance, they are of a very dark greenish black to 

 black colour, ar,d the various minerals, in some of the outcrops 

 where the rocks have been bent and folded into anticlinal and 

 synclinal folds, appear to be arranged along lines of foliation 

 parallel to the direction of the fold. In other places where the 

 rocks have not been contorted thus, no such arrangement of the- 

 component minerals occurs. Here and there throughout the 

 narrow belt are to be found instances of a commingling of this- 



